where \({n}\)  is a unit vector, here parameterized by the spherical coordinates, \(\theta \)  and \(\phi \) . While the angle \(\theta \)  determines the orientation of the principal axes of the permittivity, \(\phi \)  governs the gyrotropy of the medium [1]}. We can thus visualise these forms of lossless in–plane permittivity in terms of two real numbers, \(V_0\)  and \(V\) , and the coordinates on a sphere, \(\theta \)  and \(\phi \) . Interestingly, the eigenvalues of (REF ) (the refractive index squared) are independent of the coordinates \(\theta \)  and \(\phi \) ,
\({\rm n}^2=V_0\pm V.\) 
